This invention relates to a video signal transmission system for transmitting compressed data derived from video signals, and is more particularly suitable for application to a video signal transmission system which transmits picture data compressed by the method of orthogonal transformation, such as discrete cosine transformation and the like.
In conventional video signal transmission systems, for example, picturephones or teleconference systems, which transmit video signals of moving pictures to a distant place, video signals are coded by using the correlation between frames of video signals so that more effective transmission of significant information and effective utilization of the transmission line can be achieved.
For example, in FIG. 1, pictures PC1, PC2, PC3, . . . , which constitute a moving picture, are transmitted at time t=t.sub.1, t.sub.2, t.sub.3, . . . , respectively. In the intra-frame coding system, these pictures are coded one-dimensionally on the same line and then transmitted.
On the other hand, the inter-frame coding is carried out to improve compression efficiency by obtaining the video data PC12, PC23, . . . which represent the differences between the neighboring pictures (i.e., between PC1 and PC2, between PC2 and PC3, etc.) based on the autocorrelation of the video signal on the time axis.
Thus, the video signal transmission system can transmit the pictures PC1, PC2, PC3, . . . on a transmission line as high efficiency coded digital data whose size is extremely small compared to uncompressed picture data.
That is, as shown in FIG. 2, in a video-signal-transmission-system 1, a preprocessing circuit 2 processes an input video signal VD by frequency-band limiting, output sequence exchanging, etc. and outputs an input video data S1.
In the above operation, the video data which are sequentially supplied as input video data S1 are extracted from the frame video data FRM by the method shown in FIG. 3.
Video data of each frame FRM are divided into 2 (horizontal).times.6 (vertical) block groups GOB. Each block group GOB includes 11 (horizontal).times.3 (vertical) macro-blocks MB, and each macro-block MB has brightness signals Y.sub.1, through Y.sub.4 corresponding to 8.times.8 picture elements and color difference signal data C.sub.b and C.sub.r which are composed of color difference signals corresponding to the above stated picture elements.
In a block group GOB, macro-blocks MB are arrayed in the order of the consecutive video data, and corresponding micro-blocks are arrayed in each macro-block in the order of raster scanning.
With regard to the brightness signals, each macro-block MB has video data corresponding to 16.times.16 picture elements (Y1 through Y4) as a unit which are arranged in consecutive order in both the horizontal and vertical directions. Regarding the two color differential data corresponding to these picture elements, the amount of the data is compressed and multiplexed on the time axis. Thereafter, the data corresponding to 16.times.16 picture elements are allocated to each micro-block C.sub.r and C.sub.b.
When both the input video data S1 and the previous-frame data S2 supplied from the previous-frame memory 4 are sent to the difference-data generation circuit 3, the difference between the input video data S1 and the previous-frame data S2 is determined and inter-frame coded data S3 is generated. The difference data S3 are supplied to both the discrete cosine transformation (DCT) circuit 6 through the switching circuit 5 and to the switching control circuit 7.
The switching circuit 5 is controlled by the control signal S4, supplied from the switching control circuit 7, so that the input video signal S1 is output if it is determined that intra-field coding would give a lesser amount of transmission data. If, however, it is determined that interframe coding would give the lesser amount of transmission data, the difference data S3 is output.
The discrete cosine transformation circuit 6 for discrete cosine transforming the input video data S1 or the difference data S3 of each micro-block based on the two-dimensional correlation of the video signal. Thereafter, the transformation data S5 obtained as a result of this transformation is supplied to quantization circuit 8.
The quantization circuit 8 quantizes the transformation data S5 with a quantization step size which is determined for each block group GOB. Quantization data S6 which is output from quantization circuit 8 are supplied to the variable length code (VLC) circuit 9 and to the inverse quantization circuit 12.
The variable length code circuit 9 performs variable length coding for the quantization data S6 and outputs transmission data S7, which are supplied to the transmission buffer memory (BM) 10.
The transmission buffer memory 10 temporarily stores the transmission data S7 in a memory store and then outputs the stored data to the transmission line 11 as output data S8. Transmission buffer memory 10 further supplies the quantization control signal S9 back to the quantization circuit 8 for controlling the value of the quantization step size based on the amount of the residual data remaining in the memory store.
In other words, the transmission buffer memory 10 adjusts the amount of data which are generated as the quantization data S6 for maintaining the proper amount of residual data in the memory (in order to avoid an overflow or underflow of data).
If the amount of the residual data remaining in the transmission buffer memory 10 increases and reaches the maximum amount allowed, the transmission buffer memory 10 supplies the quantization control signal S9 for increasing the quantization step size (see FIG. 4) so that the amount of quantization data S6 decreases.
Conversely, if the amount of the residual data remaining in transmission buffer memory 10 decreases and reaches the minimum amount allowed, transmission buffer memory 10 supplies the quantization control signal S9 for decreasing the quantization step size so that amount of quantization data S6 increases.
The inverse quantization circuit 12 performs inverse-quantization on the quantization data S6 received from the quantization circuit 8 and outputs inverse quantization data S10. Thus the quantization data S6 are decoded into the inverse quantization data corresponding to such data prior to being supplied to the quantization circuit 8. The inverse quantization data S10 are supplied to the inverse discrete cosine transformation circuit 13.
The inverse discrete cosine transformation circuit 13 transforms the inverse quantization data S10, which are decoded by inverse quantization circuit 12, into the decoded video data S11 by a transformation process which is inverse to the transformation process of the discrete cosine transformation circuit 6. The decoded video data S11 are supplied to the previous-frame data generation circuit 14 and to the switching circuit 15.
Decoded video data S11 is therefore the inverse discrete cosine transformation identical to either the input video data S1 or the differential data S3 depending on switching circuit 5 (not yet transformed witch the discrete cosine transformation circuit 6).
That is, the inverse discrete cosine transformation circuit 13 produces data identical to the input video data S1 when the video signal VD is transmitted with intra-field coding, but if the video signal VD is transmitted with inter-frame coding, circuit 13 produces data identical to the difference data S3.
The previous-frame data generation circuit 14 reproduces the previous-frame video data corresponding to the data which was sent out as the output data S8, by adding the decoded video signal S11 and the previous-frame data S2 fed back from the previous-frame memory 4. This reproduced video data is sent to the previous-frame memory 4 through the switching circuit 15 and thus the pictures sent to receivers are reproduced sequentially and stored in the previous-frame memory 4.
The switching operation of the switching circuit 15 is controlled by the control signal S4 which is delayed at the delay circuit 16 for the amount of time required for the video signal VD to be processed through the aforestated circuits and supplied to the switching circuit.
However, the conventional video transmission system 1 described above has the following problems.
The amount of data generated from the quantization circuit 8 for each block group GOB are quantized based on the amount of the residual data remaining in the transmission buffer memory 10. However, the residual data exhibits two patterns; a pattern which can be easily distorted and a pattern which is not easily distorted. Since a conspicuous degradation of picture quality occurs in the pattern which can be easily distorted, a constant picture quality cannot be obtained.
In the case when the amount of transmission information fluctuates drastically at local areas of a picture, for example a picture of a rotating water wheel, a certain block of plural blocks MB constituting block group GOB contains the picture information of both fine and flat structures. Therefore, if the quantization step size is set to an averaged value, distortion concentrates locally at the block including the vanes of the water wheel wherein the patterns of the vanes might become dim or blocks on the peripheral flat portion may appear distorted.
The discrete cosine transformation system has a further disadvantage in that distortion spreads over the entire block. Therefore distortion depends on the pattern of the picture to be transmitted, and distortion fluctuates. In the high quality picture transmission system, since it is important to obtain stable and uniform picture quality independent of the content of the picture to be transmitted, distortion is a major problem.
The pictures to be transmitted may contain some areas that visually appear distorted, while other areas do not appear distorted. Distortion appears visible depending on the brightness level of each block of the picture.
In particular, the boundaries or "edges" of the image do not appear distorted, while the remaining "flat" portions appear distorted. For example, PC1 in FIG. 1 comprises an image of a torso and head. The "edges" relate to the outline of the torso and head that will appear undistorted, while the "flat" portions relate to the remainder (white) portions of the picture which may appear distorted.
For high quality transmissions, the image must be transmitted without variations in the level of distortions. However, in conventional video system transmission systems, the picture quality of transmitting images having various edges and flat portions is poor.